ACR LGT LOST HALF OF ITS ELECTRIC PWR AT HIGH ALT CRUISE AND DIVERTED TO AN ENRTE MAINT BASE.

Narrative

IN CRUISE WHEN R AC BUS LIGHT ILLUMINATED ALONG WITH R BUS ISOLATION LIGHT ILLUMINATED. MULTIPLE EICAS WARNINGS ASSOCIATED WITH UNPWRED R BUS. LOWER EICAS SCREEN AND FO'S FMC SCREEN WENT BLANK. ATTEMPTED RESETTING GENERATOR SWITCH AND GENERATOR FIELD WITH NO RESULTS. STARTED APU: HOWEVER; ITS GENERATOR WOULD NOT PICK UP THE R AC BUS LOAD. ESSENTIALLY; WE WERE DOWN TO 1 GENERATOR. OUR PROCS DON'T ALLOW US TO RESET THE BUS ISOLATION SWITCH FOR A SINGLE BUS FAILURE. SINCE THE AC BUS WAS UNPWRED; THE BATTERY CHARGER WAS INOP: THEREFORE; THE R IRS WAS DISCHARGING THE BATTERY. LEFT UNATTENDED WE WOULD HAVE LOST THE BATTERY AND GREATLY COMPLICATED OUR PROBS. WE TURNED OFF R IRS AND RERTED TO MSP FOR REPAIRS. RECOMMENDATION: SUGGEST A LITTLE MORE LATITUDE FOR CREWS TO ATTEMPT OTHER AVENUES OF APCH. WE COULD HAVE SWITCHED GENERATOR SWITCH OFF; RESET BUS ISOLATION SWITCH. THEN; THE L BUS OR APU GENERATOR COULD HAVE PICKED UP THE LOAD. ADD TO CHKLIST TO TURN OFF R IRS (IF NOT NEEDED) DURING A R AC BUS FAILURE.